1. Field of the Invention
This invention relates generally to systems wherein a liquid flows along a surface of a body. It relates particularly to a process for reducing skin friction and inhibiting the effects of liquid turbulence.
2. Description of Related Art
Skin friction drag accounts for a sizable portion of the hull drag for both surface and fully submerged marine vehicles. Reducing this drag component would have the obvious advantages of increased speed and/or efficiency. One approach to skin friction drag reduction involves the use of long chain polymer molecules, which can reduce up to 80% of the skin friction under ideal circumstances.
Although polymers generate large friction reductions, their use on marine vehicles has been limited by the large mass injection rate needed to maintain a sufficiently high polymer concentration close to the surface of the body. Turbulent diffusion in the boundary layer rapidly disperses polymer after it is injected from the body, causing much of the polymer to leave the boundary region close to the surface where the polymer drag reduction mechanism operates. Under these conditions, the amount of polymer that must be injected is uneconomical when compared to the cost of fuel needed to overcome the drag.
Wells in "An Analysis of Uniform Injection of a Drag-reducing Fluid into a Turbulent Boundary Layer," Viscous Draft Reduction (Proceedings of the Symposium on Viscous Draft Reduction), 1969, p. 361, has shown that a distributed injection of polymer over the entire surface rather than discrete injection of polymer from holes or a series of slots can produce local polymer concentration high enough to result in large drag reduction, but at less than one-tenth of the mass injection rate required by a discrete injection. While this approach allows the polymer injection rate to become economical compared to fuel costs, it also requires a uniformly porous surface over the entire body of the vehicle. This is difficult, if not totally unfeasible, to achieve on large vessels such as ships and submarines. Wells makes no mention of using the polymer in combination with a grooved surface to achieve better drag reduction.
Walsh, U.S. Pat. No. 4,706,910, discloses a method of reducing drag which uses micro-geometry longitudinal grooving of the flow surface. Walsh differs from the present invention by using the grooves alone to reduce surface drag, whereas the present invention uses surface grooves in combination with polymer to reduce the drag.
Polymers have been combined with small longitudinal grooves (riblets) in two studies by the United States Navy, and by K. S. Choi, et al of British Maritime Technology. All the experimenters were looking for possible synergistic effects between the riblet and polymer mechanisms. Their results were as follows:
(1) Both sets of Navy experimenters, Reidy and Anderson, "Drag Reduction for External and Internal Boundary Layers using Riblets and Polymers," AIAA Paper No. 88-0318; and Beauchamp and Phillips, "Riblet and Polymer Drag Reduction on an Axisymetric Body," AIAA Paper No. 88-0138, used a "polymer ocean" in their experiments and did not inject the polymer from the test surface. They neither contemplated nor provided data on the use of the riblet surface to control the turbulent diffusion of the polymer near the test surface. Instead, these studies only indicated that there were additive skin friction reductions for polymer and riblet combinations. The Navy experimenters also theorized only one injection point on the surface. As a result, large amounts of polymer would be required to overcome the rapid diffusion of polymer out of the boundary layer close to the surface. Nor did they examine the geometry of the riblets. The present invention, however, combines a study of the optimal groove measurements with multiple injection sites to minimize the amount of polymer needed to reduce drag effectively and economically by using the groove properties to limit the problem of polymer diffusion.
(2) K. S. Choi, G. E. Gadd, H. H. Pearcey, A. M. Savill, and S. Svensson, "Drag Reduction With A Combined Use of Riblets and Polymer Coating," Additive and Passive Devices: Combination Studies, 1989, p. 271, coated the riblet surface with an ablative polymer material which slowly released polymer into the flow close to the surface. The slow release of the polymer from the surface was entirely a result of the nature of the polymer ablative coating, not the riblet geometry. Choi neither contemplated nor provided data pertaining to the use of the riblet geometry to control the polymer diffusion. Not only did this technique produce much less than optimum polymer drag reduction (10% vs. 80% possible, indicating inefficient release and dispersion), but over time the sharp groove peaks will be worn down. As a result, the grooves will become much less effective in reducing drag. In the present invention, polymer is injected into the grooves, not coated on the surface. The present invention is specifically designed to use the riblet geometry to control release of polymer after it has been injected.